The present invention relates to an optoelectronic measuring system comprising at least two transmission light sources, a compensation light source, a receiver for receiving the light radiated by the transmission light sources and the compensation light source and for converting received light signal into an electrical receiving signal, and a control and evaluation unit. The generation of a compensation control signal to control the compensation light source serves to compensate for scattering light, so that a measurement of light reflected by an object that is conducted to detect the object can be completed without the need to consider scattering light.
Optical measuring systems with scattering light compensation are known for example from EP 0 706 648 B1, which describes an optical system for detecting a change in the reflection from an object. With this system, measurements can be taken in the presence of scattering light and changes in the scattering light or ambient light do not affect the measurement value. Such a system is described using the example of a windshield wiper sensor, with which raindrops falling on the windshield are detected. Systems of such kind may also serve as the basis for creating other optical measuring arrangements for evaluating optical couples. Examples of this are reflection light barriers or transmitted light barriers.
Such a measuring system comprises two measurement paths, each of which includes a transmission light source and a shared receiver. One of the two initially equivalent transmission light sources will hereafter also be designated the compensation light source. Each measurement path has an optical couple that is derived from the ratio of received to transmitted energy. While both light sources emit radiation, particularly light, in the visible and invisible ranges, the receiver detects the light and radiation coupled via both measurement paths that are reflected by objects or other surfaces or are transmitted by the respective media in the measurement paths. A measurement signal that is independent of the influences of ambient light and other interference factors acting on the receiver may be obtained by comparing the two measurement paths.
The measurement arrangement comprising the two transmission light sources and the receiver is operated via the associated control and evaluation unit in such manner that the transmission light sources transmit alternatingly at the frequency of a shared clock generator. The light components of both transmission light sources modulated in this manner are then contained in the receiving signal, weighted by the coupling factors of the respective measurement paths. The evaluation unit demodulates the received signal again into the two signal components assigned to the measurement paths synchronously with the clock generator. The difference between the two signal components is now balanced to zero using a compensation measuring method by adjusting the amplitude of the control signals from the transmission light sources. In the compensated, that is to say the adjusted state, the quantity of radiation received is the same for both measurement paths, and the clock-synchronous alternating component is thus also zero. The ratio between the control signals for the two transmitted light sources forms the actual useful signal of the measuring device.
It is known from the related art that if considerably more than two measurement paths are needed, another transmission light source must be installed for each measurement path. The measuring arrangement is then evaluated in a sequential multiplex mode, so that exactly two measurement paths can be adjusted with respect to one another in a series of consecutive operations. However, as more and more measurement paths are needed, processing cycles also become longer and longer, slowing down the measuring system.
The system described in EP 0 706 648 B1 with an analog adjustment system is enhanced in EP 1 671 160 B1 and adapted for a digital system to create a technically simpler apparatus and achieve a high degree of sensitivity.
In DE 103 00 223 B3, a compensation light source is used as well as the transmission light sources. Only the compensation light source is adjusted, whereas the transmission light source used to monitor an optical measurement path is either not adjusted or is controlled by considerably simpler means.